For the purpose of developing a new type of photo-and electro-active materials, we have investigated the synthesis of oligothiophenes with well-defined structures and novel polymers containing pendant oligothiophenes, and their electrical, photoelectrical and electrochromic properties.In order to elucidate the relationship between electrical conductivities and the length of pi-conjugation in oligothiophenes, we have investigated electrochemical doping of alpha-ethyl-disubstituted oligothiophenes, and electrical properties of resulting radical-cation salts. The electrochemically doped thiophene tetramer and pentamer were found to exhibit room-temperature conductivities of 10^<-7> and 10^<-3> S cm^<-1> with activation energies of 0.5 and 0.1 eV,respectively.In order to develop a new class of materials for photoelectric conversion, we have studied oligothiophenes for use in photovoltaic devices. The conversion efficiency of the ITO/perylene pigment (400A)/thiophene octamer (300A)/Au cell was ca. 0.6 % for the incident white light (10-200 mW cm^<-2>) through the ITO electrode. The present study shows that oligothiophenes constitute a new class of promising photoactive materials that function as p-type semiconductors for use in photovoltaic devices.We have investigated the synthesis and properties of novel polymers containing pendant oligothiophenes with well-defined structures, which are expected to have unique properties characteristic of both pendant oligothiophenes and non-conjugated polymer backbone. The electrical conductivity of electrochemically doped polymers was found to increase with the increase in the length of pendant oligothiophenes. The electrochemically doped polymers were found to undergo a reversible, clear color change The present study shows that electrochemically doped polymers containing pendant oligothiophenes with well-defined structures constitute a novel class of potential electrochromic materials.